I2C: mv64xxx: fix error handling for request_irq()

Propagate the error code from request_irq() rather than ignoring it
entirely.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>